While the percentage of oxygen in the earth’s atmosphere is the same at all altitudes, the density of the air and thus the amount of oxygen available to human lungs decreases at higher altitudes. At these locations decreased atmospheric pressure causes there to be fewer molecules per breath. In the 17th century, this pressure function came to be understood when it was discovered that air was not a void or a weightless ether but had mass on which the earth’s gravity was exerting a pull. With this information it was possible to understand that the earth’s “surface” was not the top of something but the bottom - the floor of a massive ocean of air.”